The present invention relates to neurology and pharmacology, and more specifically to drug treatments that can prevent or reduce the brain damage caused by Alzheimer's disease.
The following Background sections provide introductory information on (1) neurotransmitter receptors in the brain; (2) mechanisms by which these transmitter and receptor systems may contribute to neuronal degeneration in Alzheimer's disease; and (3) certain types of drugs that can be used to prevent or reduce neuronal degeneration in patients suffering from Alzheimer's disease.
The following Background sections are not strictly limited to prior art. The extremely complex (and apparently contradictory and paradoxical) neurological systems and processes involved in Alzheimer's disease have stubbornly confounded the efforts of literally thousands of highly skilled researchers and physicians, for decades. Accordingly, the following narrative is an effort to explain, as clearly and logically as possible, what is happening inside the brain of someone suffering from Alzheimer's disease, and how various neurological networks interact with each other in apparently paradoxical ways. Substantial parts of this explanation come from the Applicants' recent research discoveries. Because some of these recent discoveries involve neurological processes that occur naturally, inside the brain, they are discussed in the Background narrative. However, these recent discoveries by the Applicants should not be regarded as prior art.
Glutamate (GLU) and Neuronal Glutamate Receptors
Glutamate (sometimes abbreviated as GLU) is one of the 20 common amino acids used by all living cells to make protein. Glutamate is the ionized form of glutamic acid; the ionized form is the predominant form in neutral solutions, at pH 7.
In addition to its role as a building block for proteins, glutamate plays an entirely different and crucial role in the central nervous system (CNS) of higher animals, including mammals and birds. Although much of the discussion that follows is equally true for the spinal cord, which is part of the CNS, this discussion focuses on the brain, since that is where the damage and degeneration occurs in patients suffering from Alzheimer's disease.
Within the brain, glutamate serves as the predominant excitatory transmitter molecule which carries signals between nerve cells (e.g., Olney 1987; full citations to books and articles are provided below). In a brief overview, this process can be summarized as follows. At a neuronal synapse (i.e., a signal-transmitting junction between two nerve cells), a molecule of glutamate is released by the signal-transmitting neuron. The glutamate molecule enters the fluid in the gap between the two neurons, and it rapidly contacts the exposed portion of a "glutamate receptor" on the surface of the signal-receiving neuron.
As used herein, "receptor" refers to a macromolecular binding site (usually a protein, which may also be glycosylated or phosphorylated) which is at least partially exposed on the surface of a cell, and which has specific and limited affinity for one or more fluid-borne molecules, called "ligands" (these usually are neurotransmitters or hormones). When a ligand contacts an appropriate receptor, a brief binding reaction occurs which causes a cellular response, such as opening of an ion channel, which leads to activation and depolarization of the neuron. Most receptor molecules are proteins which straddle the membrane of a cell,, with an external portion for binding reactions, and an internal portion which helps carry out the cellular response that occurs when the receptor is activated by a ligand.
This is not a rigid definition, and different scientists sometimes use the term "receptor" inconsistently. For example, they may either include or exclude various additional components, such as an ion channel which is opened or closed by a receptor. All of the glutamate receptors relevant to the present invention are associated with ion channels, and therefore are referred to as "ionotropic" receptors.
In pharmacological terminology, an "agonist" is a molecule which activates a certain type of receptor. For example, glutamate molecules (and certain drugs such as NMDA, as described below) act as agonists when they excite EAA receptors. By contrast, an "antagonist" is a molecule which prevents or reduces the effects exerted by an agonist at a receptor.
Upon being activated ("excited") by a glutamate molecule, a glutamate receptor protein changes its conformation, in a manner which briefly opens an ion channel that serves as a conduit through the cell membrane. Calcium (Ca.sup.++), sodium (Na.sup.+), and certain other types of ions rapidly flow through the ion channel when it is briefly opened, thereby altering several ionic gradients that normally exist across the membranes of neurons at rest. This activates (stimulates) a neuron, causing it to release its own neurotransmitters at other ("downstream") synapses, thereby transmitting signals to still other neurons.
To reset the mechanism and get the transmitting and receiving neurons back to a resting/ready condition, where both neurons are ready to handle another nerve signal, the ion channel quickly closes, and the glutamate receptor protein on the signal-receiving neuron releases the glutamate molecule. The glutamate molecule floats back into the synaptic fluid between the neurons, and a molecular transport system quickly intercepts it and transports it back inside the transmitting neuron. The signal-receiving neuron activates a set of molecular pumps, which rapidly transport calcium and sodium ions (which had entered the cell though the glutamate-controlled ion channel) back out of the neuron to regain a "polarized" condition, so that it will be ready to receive another nerve signal.
This entire set of chemical actions--release of glutamate by a transmitting neuron, activation and depolarization of a signal-receiving neuron, release of the glutamate transmitter molecule by the receptor protein, clearance of the free glutamate from the synaptic fluid, and restoration of the polarized/ready state in the signal-receiving neuron--is extraordinarily rapid. All of these steps, together, occur within a few milliseconds.
Since glutamate is an amino acid that can function as an excitatory neurotransmitter inside the brain, it is often called an "excitatory amino acid" (EAA). Another type of amino acid, aspartate (the ionized form of aspartic acid), can also function as an excitatory amino acid in the brain; therefore, glutamate receptors are sometimes referred to as "EAA" receptors, since they can be triggered by either of two amino acids (glutamate or aspartate). However, glutamate is used much more widely than aspartate as a neurotransmitter, and "EAA receptors" are referred to herein (and by most scientists) as glutamate or GLU receptors.
Types of GLU Receptors: NMDA and non-NMDA Receptors
There are three distinct types of ionotropic glutamate receptors in the mammalian central nervous system (as well as a "metabotropic" glutamate receptor, which is not of interest herein). Although all three GLU receptor types are normally triggered by exactly the same EAA neurotransmitters in the CNS (i.e., glutamate or aspartate molecules), these three different subtypes of glutamate receptors have been found by researchers to have different binding properties, when certain types of artificial drugs are used as probes to study neuronal activity.
One major class of GLU receptors is referred to as NMDA receptors, since they bind preferentially to NMDA, which is n-methyl-D-aspartate. This analog of aspartic acid normally does not occur in nature, and is not present in the brain; it is, however, a useful probe drug which is widely used by neurologists to study and differentiate the roles of NMDA and non-NMDA receptors. When molecules of NMDA contact neurons having NMDA receptors, the NMDA strongly activates NMDA receptors, and acts as a glutamate agonist, causing the same type of neuronal excitation that glutamate causes.
The second class of glutamate receptors is called "kainic acid" (KA) receptors, since they are activated by kainic acid, another artificial drug that does not normally occur inside the brain.
The third class of GLU receptors is referred to herein as AMPA receptors; they are activated by both quisqualic acid (and its ionized form, quisqualate) and by alpha-amino-3-hydroxy-5-methyl-4-isoxazole (abbreviated as AMPA). Until the mid-to-late 1980's, AMPA receptors were called quisqualate (QUIS) receptors; however, quisqualate also activates a different type of receptor called a metabotropic receptor, so the recent trend is to call QUIS-type EAA receptors by the name "AMPA" receptors.
KA receptors and AMPA receptors are more closely related to each other (both structurally, and by higher levels of cross-affinity to various drugs) than to NMDA receptors. In addition, they could not be distinguished from each other for several years after it was recognized that there were NMDA-affinity receptors as well as other classes of glutamate receptors. For both of these reasons (scientific, and historical), KA and AMPA receptors are often referred to collectively as "non-NMDA" receptors.
The NMDA receptor complex (which includes an ion channel) has a number of distinct binding sites (also called recognition sites), where several different substances can bind to and modify the ion-channel-opening actions of glutamate. Thus, there are several different types of NMDA antagonists; each type is characterized in terms of the binding site with which it interacts.
"Competitive" NMDA antagonists compete with glutamate at the glutamate binding site (which is also the NMDA binding site). The action of glutamate at this site promotes opening of the ion channel to allow Na.sup.+ and Ca.sup.++ ions to flow into the cell. Competitive antagonists block the action of glutamate at this site, and prevent opening of the ion channel; thus, they are often referred to as "closed channel blockers." Competitive NMDA antagonists being developed by drug companies are usually given acronyms or code numbers; these include, but are not limited to, compounds such as CPP (Boast 1988), D-CPP-ene (Herrling 1994), CGP 40116 (Fagg et al 1989), CGP 37849 (Fagg et al 1989), CGS 19755 (Boast 1988 and Grotta 1995), NPC 12626 (Ferkany et al 1989), and NPC 17742 (Ferkany et al 1993). Other competitive NMDA antagonists include D-AP5 (D-2-amino-5-phosphonopentanoic acid), D-AP7, CGP 39551 (D,L(E)-2-amino-4-methyl-5-phosphono-3-pentenoic acid carboxyethyl ester), CGP-43487, MDL-100,452, LY-274614, LY-233536, and LY-233053.
Regrettably, even though these drugs held early promise for possibly reducing brain damage caused by stroke, cardiac arrest, etc., most of these drugs have been shown to cause pathological changes in certain regions of the mammalian brain, in lab animals (Olney et al 1991; Hargreaves et al 1993). All of these drugs that have been tested in adult humans have also been shown to cause psychotomimetic reactions, such as hallucinations, which suggest that similar types of damage in certain vulnerable regions of the brain may also occur in humans when competitive NMDA antagonists are administered (Grotta 1995; Herrling 1994; Kristensen et al 1992).
There are also other sites in the NMDA receptor complex, located outside the ion channel, where glycine or certain types of polyamines can bind. Binding of glycine and polyamines to these sites exerts a cooperative action that assists glutamate in opening the ion channel. Accordingly, it is hoped and believed by some that drugs which block the glycine or polyamine sites may have neuroprotective actions which are comparable to, but somewhat milder than, competitive antagonists which act at the glutamate binding site. Glycine and polyamine site antagonists include but are not limited to kynurenic acid, CNQX, DNQX, 6,7-DCQX, 6,7-DCHQC, R(+)-HA-966, 7-chloro-kynurenic acid, 5,7-DCKA, 5-iodo-7-chloro-kynurenic acid, MDL-28,469, MDL-100,748, MDL-29,951, L-689,560, L-687,414, ACPC, ACPCM, ACPCE, ACEA 1021, ACES 1031, arcaine, diethylenetriamine, 1,10-diaminodecane, 1,12-diaminododecane, ifenprodil, and SL-82.0715. For reviews and citations, see Carter et al 1988, Rogawski 1992, Massieu et al 1993, Keana et al 1995, and Warner et al 1995.
Within the NMDA receptor ion channel, there is a site where phencyclidine (PCP) and several other drugs (including dizocilpine, ketamine, tiletamine, and CNS 1102) bind selectively. When these agents bind to the PCP site in the ion channel, they block ion flow through the channel, even if the channel otherwise remains open: thus, drugs that block activity at NMDA receptors by binding to the PCP site are sometimes referred to as "open channel blockers".
Dizocilpine is the most selective and effective non-competitive NMDA antagonist ever discovered; it is powerful and highly selective at the PCP binding site. The full chemical name is (+)-5-methyl-10,11-dihydro-5H-dia,d!cyclohepten-5,10-imine. The maleate salt of dizocilpine is commercially available to researchers under the name MK-801, and MK-801 has been investigated extensively for use as an antiepileptic and for preventing damage due to cerebral ischemia. However, it has been shown, even at relatively low doses, to produce pathological changes in cerebrocortical neurons in adult rats (Olney et al 1989).
Phencyclidine is a dissociative anesthetic, formerly used in human and veterinary medicine, that is illicitly abused as a hallucinogenic drug under the street name "angel dust". This drug can induce a psychosis which is clinically indistinguishable from schizophrenia, and it has been shown at relatively low doses to produce pathological changes in various corticolimbic regions of the adult rat brain (Olney et al 1989, Corso et al 1994).
Ketamine is a dissociative anesthetic that is currently being used in human anesthesia, and is the only NMDA antagonist that is currently being used for anesthetic purposes. It is suitable for human anesthesia because it has an exceedingly short duration of action (usually about 15 minutes), which assures that its effects on the CNS, including adverse CNS effects, can be rapidly reversed by termination of ketamine administration. Despite its short duration of action, it occasionally produces an "emergence" reaction during recovery from anesthesia that is characterized by unpleasant dreams, confusion, agitation, hallucinations, and irrational behavior. Ketamine has recently been studied for its psychotomimetic effects and has been described as an agent that produces symptoms in normal humans that are indistinguishable from the symptoms of psychosis and thought disorder seen in schizophrenia (Krystal et al 1994). Ketamine also has been shown to cause pathological changes in the cerebral cortex of adult rats (Olney et al 1989).
Tiletamine, also used in veterinary medicine as an anesthetic, is another non-competitive NMDA antagonist which acts at the PCP binding site. It has also been shown to cause pathological changes in the cerebral cortex of adult rats (Olney et al 1989).
Toxic Effects of Excessive NMDA Receptor Activity; Use of NMDA Antagonists to Reduce Excitotoxic Damage
Excessive activation of NMDA receptors by endogenous glutamate is thought to play a major role in a number of important CNS disorders. In an acute crisis such as a stroke or CNS trauma, and in certain other events such as severe epileptic seizures, the cellular transport mechanism that removes glutamate almost immediately from the synaptic fluid, and pumps it back inside a neuron for subsequent re-use, can run out of energy to drive the glutamate clearance process. If this occurs, excessive glutamate begins to accumulate in the synaptic fluid between neurons. If glutamate molecules are not being removed from synapses at adequate rates, they begin to repeatedly and persistently excite glutamate receptors on signal-receiving neurons. This drives the receptor-bearing neurons into a state of hyper-excitation which can kill the neurons, through a process called "excitotoxicity" (e.g., Olney 1990, Choi 1988, Choi 1992).
Excessive activity at NMDA receptors can also severely aggravate neuronal damage caused by trauma (mechanical injury) to the brain or spinal cord. Many trauma victims suffer from a dangerous and potentially lethal increase in intracranial pressure, which involves water flowing into neurons in an effort to sustain osmotic balance as charged ions flow into the neurons during neuronal excitation. Elevated intracranial pressure is a major cause of morbidity and mortality in CNS trauma victims, and NMDA antagonists are potentially useful in reducing intracranial pressure following such crises.
As used herein, the term "acute insult to the central nervous system" includes short-term events which involve or pose a threat of neuronal damage mediated by glutamate excitotoxicity. This includes ischemic events (which involve inadequate blood flow, such as a stroke or cardiac arrest), hypoxic events (involving inadequate oxygen supply, such as drowning, suffocation, or carbon monoxide poisoning), trauma in the form of mechanical or similar injury, certain types of food poisoning which involve an excitotoxic poison such as domoic acid, and seizure-mediated neuronal degeneration, which includes several types of severe epileptic seizures. NMDA antagonists can help to protect neurons in the CNS against such damage (e.g., Olney 1990; Choi 1992). Accordingly, a number of NMDA antagonists (i.e., drugs that can suppress glutamate's excitatory activity at NMDA receptors) are being studied by several major pharmaceutical companies.
Despite the intense interest and research in NMDA antagonist drugs, for roughly a decade now, no NMDA antagonists have been approved for human use, except in very limited clinical trials (this excludes ketamine, which was approved for use as a short-acting anesthetic, during surgery, before the toxic side effects of NMDA antagonists were recognized). Presumably, this is because of the toxic side effects that these drugs cause in certain regions of the brain.
Various types of "safener" agents have been known for years, which can prevent those toxic side effects (see, e.g., Olney 1991, and U.S. Pat. No. 5,034,400 (Olney 1991)). However, except possibly in small clinical trials, NMDA antagonists still are not being used on humans, despite the current knowledge about safener agents.
In addition to neuronal damage caused by acute insults, excessive activation of glutamate receptors may also contribute to more gradual neurodegenerative processes leading to cell death in certain chronic neurodegenerative diseases, including amyotrophic lateral sclerosis (Lou Gehrig's disease), Parkinson's disease and Huntington's chorea (Olney 1990). It is considered likely, by many researchers in this field, that NMDA antagonists may someday prove useful in the therapeutic management of such chronic diseases.
As noted below, several neurological researchers (including the Applicants) have proposed that excessive activation of NMDA receptors may also play a role in Alzheimer's disease, a neurodegenerative disease that is discussed in some detail below. However, it now appears (based on recent discoveries by the Applicants, as disclosed herein) that in the very early stages of Alzheimer's disease, there is a shift from one form of NMDA receptor dysfunction, to an entirely different form of NMDA receptor dysfunction. This will be explained below.
NR/Hypo as a Drug-Induced Phenomenon: Both Beneficial and Detrimental
A condition of NMDA receptor hypofunction (NR/hypo) can be created, inside the brains of laboratory animals, by administering an NMDA antagonist drug (i.e., a drug which blocks or reduces activity at NMDA receptors). When NR/hypo is deliberately created, using NMDA antagonist drugs, it can have important beneficial effects in protecting brain tissue against acute excitotoxic damage due to a crisis such as a stroke, cardiac arrest, or brain trauma. Therefore, pharmaceutical companies have devoted hundreds of millions of dollars to research funding, in an effort to develop safe NMDA antagonist drugs that can be used to treat people suffering from such crises.
However, despite their potential benefits, NMDA antagonists have serious detrimental side effects. As described in Olney et al 1989b and in U.S. Pat. No. 5,034,400 (Olney 1991), they can injure and even kill neurons located in a portion of the brain known as the posterior cingulate or retrosplenial (PC/RS) cortex, and in certain other cerebrocortical and limbic regions of the animal brain.
These toxic side effects, caused by NMDA antagonists, can be evaluated and measured directly, in brain tissue from lab animals that have been sacrificed. Such tests are most commonly done on rats. Briefly, in sacrificed animals which have been administered an NMDA antagonist drug (such as MK-801) without an accompanying safener drug, several pathological changes become fairly obvious and easily detectable in neurons located in the PC/RS cortex region of the brain. Such changes include (1) the formation of vacuoles (i.e., small bag-like sacks or organelles which are empty of the type of cellular structures that normally fill other organelles inside a cell cytoplasm; (2) mitochondrial damage or dissolution; and (3) the induction and expression of so-called "heat shock" proteins. All three of these measurable indicators (vacuole formation, mitochondrial damage, and expression of heat shock proteins) can be regarded as manifestations of serious damage to affected cells, indicating major disruption or derangement of a cell's normal structure and functioning. These cellular changes can also be correlated with behavioral abnormalities in lab animals, such as seizures, catatonic withdrawal, and abnormal responses to conventional stimuli.
In living human patients, since direct examination of brain tissue is usually not feasible prior to death, the appearance of hallucinations or other psychotomimetic symptoms (such as severe disorientation or incoherence) is regarded as a warning sign that similar types of neuronal damage may be occurring in the human brain, when NMDA antagonists are used.
Thus, a major obstacle to the use of NMDA antagonists as therapeutic drugs in humans lies in their potential for inducing toxic side effects inside the brain, including neuronal damage and even neuronal death, in certain vulnerable regions of the brain.
Safener Drugs for Preventing Toxic Side Effects of NMDA Antagonists
It has been discovered, largely by the Applicants herein, that several types of drugs can act as "safener" agents to reduce or prevent the toxic side effects caused by NMDA antagonists. Safener drugs that have previously been described are listed below. Not all of these drugs are suitable for long-term administration; however, this brief listing is provided for completeness.
(1) Anticholinergic drugs which block the muscarinic class of cholinergic receptors. These are described in more detail in Olney et al 1991, and in U.S. Pat. No. 5,034,400 (Olney 1991).
(2) Benzodiazepine drugs which can increase ("potentiate") the effects of a naturally-occurring inhibitory neurotransmitter called gamma-amino-butyric acid (GABA) have some efficacy in preventing toxic side effects of NMDA antagonists. However, even in very high dosages, these drugs are not completely effective. They can only act to enhance the effects of naturally-occurring GABA, and they become ineffective when inadequate supplies of naturally-occurring GABA are not present. Examples include diazepam, which is sold under the trademark VALIUM, and its various analogs.
(3) Other drugs which can act directly at GABA type A (GABA.sub.A) receptors, to open the chloride ion channel even in the absence of naturally occurring GABA, can block the toxic side effects of NMDA antagonists much more effectively than benzodiazepine drugs (see U.S. Pat. No. 5,474,990, Olney 1995). This class of drugs, called "direct GABA agonists", includes certain barbiturates such as secobarbital and pentobarbital. It also includes certain anesthetics such as isoflurane and halothane, which are administered by inhalation, as well as propofol, an intravenous anesthetic.
(4) Drugs that can bind to a class of receptors called sigma receptors, as described in Farber et al 1993. These drugs include di(2-tolyl)guanidine and rimcazole, which are relatively selective for sigma receptors, as well as other drugs such as haloperidol, thioridazine and loxapine, which interact with dopamine receptors as well as sigma receptors.
(5) Drugs that act as agonists at a class of receptors called alpha-2 adrenergic receptors. Drugs in this class include clonidine, p-iodoclonidine, guanabenz, guanfacine, xylazine, and lofexidine (Farber et al 1995a and 1995b).
(6) Certain types of antipsychotic agents, including clozapine (Farber et al 1993), olanzapine, and fluperlapine (Farber et al 1996). In addition to acting at dopamine, serotonin, and norepinephrine receptors, these drugs also bind to and suppress activity at muscarinic receptors. Accordingly, these drugs can be grouped together with the anti-cholinergic (muscarinic antagonist) drugs mentioned in item #1, above.
In addition to the foregoing classes of drugs, which have been publicly identified as being capable of blocking the toxic side effects of NMDA antagonists, the Applicants' recent research has also identified certain additional types of drugs that can accomplish that same goal, but which have not previously been publicly disclosed as having that capability. These additional classes of drugs are identified and discussed below, following the Summary of the Invention.
Before proceeding further, another phenomenon needs to be analyzed, involving inhibitory neurotransmitter receptor systems and a potentially pathological process known as "disinhibition".
Inhibitory/Excitatory Transmitter Interactions; Disinhibition
It was mentioned above that in the brain, glutamate is the principle type of excitatory neurotransmitter. Several other types of neurotransmitters also need to be discussed, since the interactions between glutamate and these systems are important to this invention.
Another major excitatory neurotransmitter is acetylcholine (abbreviated as ACh). Like glutamate receptors, there are several different types of ACh receptors on neurons in the brain. These are generally divided into "muscarinic" and "nicotinic" classes of ACh receptors. The muscarinic class is further subdivided into m1, m2, m3, m4, and m5 subclasses.
When a molecule of ACh contacts an ACh receptor on a neuron, this triggers a signal transduction process involving certain "second messenger" systems within the neuron, the net result being that a higher state of electrical activity is induced; this is another way of saying that the neuron is excited by ACh. In addition, at some muscarinic receptors, especially m2 receptors, ACh may have predominantly inhibitory or autoregulatory effects, instead of excitatory effects.
In the mammalian brain, there also is a type of neurotransmitter receptor system referred to as the "sigma" receptor system. Although this system has been known for many years, progress has been slow in identifying the endogenous transmitter molecule that activates this system. Recent evidence indicates that a certain peptide molecule that is abundantly contained in certain CNS neurons, called neuropeptide Y (NPY), has an important role in modulating the IV function of sigma receptors. The effects of NEIY on the sigma receptor system appear to be mainly excitatory.
Accordingly, in the brain circuitry that is relevant to the present invention, there are three excitatory transmitter receptor systems (glutamate, ACh, and sigma) and three corresponding excitatory transmitter molecules (glutamate, ACh and NPY, respectively).
In addition to these three excitatory transmitter systems, there are also transmitter systems in the CNS that are primarily, or in some cases exclusively, inhibitory. These inhibitory systems are absolutely essential to proper functioning of the brain. In a simplified summary, they can be regarded as serving two distinct functions. First, they help a neuron quickly restore itself to a "resting/ready" condition, so that it will be ready to receive the next nerve signal from other neurons. And second, they help suppress or "tune out" activity caused by unwanted impulses. This is analogous to a TV or radio receiver, which cannot function properly unless it can be tuned to a single channel or station, so that it suppresses and ignores the competing signals from dozens or hundreds of other transmitting stations that may be broadcasting in the area.
Another way to understand both the importance and the mechanism of inhibitory neurotransmitters and receptors is to think in terms of gatekeepers, which establish threshold values. If a weak, low-level signal that does not reach a necessary threshold strength tries to activate a neuron, the gatekeeper will block it. However, if an incoming signal or stimulus reaches or surpasses the threshold level, the gatekeeper will let it through. It will then trigger what is, in effect, an on/off switch which controls the activation, firing, and depolarization of the neuron. This inhibitory/gatekeeping function is essential for reducing spurious and unwanted nerve cell activations in the brain, so that coherent, meaningful patterns of signals (stimuli, thoughts, memories, etc.) can be handled properly and effectively by the brain.
The predominant inhibitory transmitter in the brain is GABA (the acronym for gamma-amino butyric acid). This inhibitory transmitter has important interactions with the glutamate excitatory system in many neural circuits within the CNS. Neurons that contain and release GABA as an inhibitory transmitter are called GABAergic neurons.
As is discussed in more detail below, there is phenomenon involving the GABA system known as "disinhibition" which can play an important role in neuropathological disease processes. The GABA system normally is constantly active in maintaining a certain level of inhibitory tone which exerts a restraining influence on the major excitatory pathways (including both glutamatergic and cholinergic systems) in the brain. The GABA system maintains a state of constant inhibitory activity by virtue of being steadily driven by glutamate. This glutamate is released by neurons at synapses which use NMDA receptors on the "incoming" or "upstream" surfaces of the GABA-releasing neurons. Thus, paradoxically, glutamate is the driving force behind a multi-component inhibitory system that functions to restrain glutamate and other excitatory systems so that they will not over-excite and damage other neurons in the brain. This system is discussed in more detail below, and is schematically depicted in FIGS. 1 and 2.
It follows, if brain systems are organized in this manner, that if the NMDA receptors on the GABA-releasing neurons, through which glutamate drives the inhibitory restraining system, are impaired or destroyed (i.e., if an NR/hypo condition is created inside the brain), the restraining action will be abolished and the excitatory systems that were being restrained will be released from inhibition ("disinhibited"). Unrestrained excitatory activity will then serve as a pathological mechanism that can cause neuronal degeneration in the brain circuits that have been disinhibited.
NR/Hypo as a Disease Mechanism
Before analyzing the correlations between NR/hypo and Alzheimer's disease, it should be noted that there has been some interest in NR/hypo as a disease process, but this interest has been focused upon schizophrenia and not Alzheimer's disease. These two diseases are so totally different, in their symptoms and manifestations, that various published suggestions that NR/hypo is involved in schizophrenia (e.g., Olney and Farber 1995) would be regarded, by most neurologists, as teaching away from possible involvement as a causative mechanism in Alzheimer's disease. This may be one of the reasons that (apparently) no other neurological researchers have noticed any correlations between NR/hypo and Alzheimer's disease.
Schizophrenia is not of interest herein; this current invention solely involves the treatment and prevention of Alzheimer's disease. To the best of the Applicants' knowledge and belief, no one else has ever suggested that NR/hypo might be a causative mechanism in Alzheimer's disease.
Alzheimer's disease
Alzheimer's disease is a progressive neurodegenerative disorder which mainly affects people over the age of 60. Memory loss and general cognitive deterioration, often leading to disorientation, extreme forgetfulness, and an inability to care for one's self or one's affairs, are the primary symptoms; however, these terms are inadequate to convey the mental devastation, family suffering, and economic and social costs that the disease inflicts on its victims and their families when elderly parents can no longer function, travel in society, or (in many cases) even recognize their own children.
The disease does not immediately become clear and apparent in its early stages, and only becomes evident over a prolonged span of time, which may cover years. This is especially true since nearly everyone suffers from some level of memory loss as they age into their sixties and seventies, and this type of normal memory loss can mask the early onset and recognition of Alzheimer's disease. Therefore, it is impossible to accurately determine how many people suffer from Alzheimer's disease. However, it has been estimated that tens of millions of elderly people suffer from Alzheimer's disease at some level, and the numbers continue to grow as advances in other areas of health care prolong life expectancies and allow Alzheimer's disease to manifest itself more frequently. There can be no doubt that Alzheimer's disease is one of the most important and costly health problems, among the elderly.
Neuropathological damage which can be measured in the brain after death include atrophy (shrinkage, loss of mass, and loss of healthy, viable tissue) in the forebrain, massive loss of neurons and synaptic complexes, and the presence in many brain regions of neurofibrillary tangles and amyloid plaques. Interestingly, the amyloid plaques have a different distribution than the neurofibrillary tangles; however, no one previously has offered a satisfactory explanation for those distributions.
Various diagnostic tests and analyses have been developed for diagnosing Alzheimer's disease. These tests can be divided into three major categories: genetic analysis, cognitive testing, and brain scans.
The first category, genetic analysis, involves biochemical analysis of genes from cells (e.g., blood cells) taken from a patient or suspected patient. Certain isoforms (or alleles) of certain genes are known to substantially increase the risk that a person carrying that gene will manifest the symptoms of Alzheimer's disease; an unfavorable isoform may also be associated with an earlier onset of symptoms. In addition, genetic mutations that invariably cause Alzheimer-type dementia to occur within certain families, usually with early onset, have been discovered. Genetic analysis is discussed in more detail below.
The second major category involves cognitive testing, in which the patient responds to questions which evaluate perception, memory, and analytical abilities. Various such tests, which are sufficiently sophisticated to be able to detect the earliest outward manifestations of Alzheimer's disease, are described in articles such as Morris 1993, and Morris et al 1988.
The third major category involves neuro-imaging (also called brain scans). The major type that was first developed, called CAT scans (CAT is the acronym for computerized axial tomography), has been superseded by magnetic resonance imaging (MRI), which provides better and clearer pictures with finer resolution. Both of these scanning types reveal structural details inside the brain; they do not require administration of special drugs to the patient, and they only reveal static structures, rather than areas where neuronal activity is occurring.
Two other types of scans can analyze brain activity, rather than merely static structures. PET scanning (PET stands for positron emission tomography) involves administration of special drugs that are usually labelled with short-lived radioactive isotopes. These drugs create highlighted areas in a scan, and the highlighted areas indicate areas of increased neuronal receptor binding or neuronal activity, inside the brain. A more recent and improved approach is called magnetic resonance spectroscopy; like PET scans, it involves administration of special drugs to a patient, and these drugs highlight areas of elevated binding or activity in the brain.
There are many published reports indicating that various abnormalities in the brains of Alzheimer patients (including very early stage Alzheimer patients) can be detected via brain scans. Review articles which summarize and cite such reports include Jagust 1996, Rossor et al 1996, and Smith 1996.
In summary, an enormous amount of research effort and funding has been spent on Alzheimer's disease, and a number of sophisticated diagnostic methods have been developed. Cognitive testing methods can diagnose Alzheimer's disease in its very earliest clinical stages, and by genetic analysis and neuro-imaging performed prior to onset of symptoms, it is often possible to identify individuals who are at increased risk of developing Alzheimer's disease, and in some cases to predict with certainty that specific individuals are destined to be afflicted with the disorder.
Despite extensive research, which has been continuing for decades, no one has previously managed to clearly decipher the etiologic mechanisms of Alzheimer's disease. In addition, with a single very limited exception (which involves a drug called tacrine, which provides only a brief and equivocal respite from the onslaught of Alzheimer's disease, as discussed below), no one has previously been able to specify any drug treatment which can actually reduce, retard, or prevent the ongoing neurological devastation caused by Alzheimer's disease. As discussed below, all other treatments are merely palliative, i.e., they merely try to make an Alzheimer's patient more comfortable while the damage continues inside his or her brain.
Prior to this invention, the two most prominent schools of thought regarding the neurodegenerative changes in Alzheimer's disease have involved genetics, and excitotoxicity. The positions maintained by these two differing schools of thought can be briefly summarized as follows.
Genetics School
As noted above, geneticists have identified several specific genetic factors which they believe may be of etiological significance in Alzheimer's disease. The two types of genes that are of greatest interest in Alzheimer research are: (1) genes which encode a protein known as apolipoprotein-E; and (2) genes that encode amyloid precursor proteins which, when enzymatically cleaved, give rise to beta amyloid, a protein that is an important ingredient of amyloid plaques.
Review articles which survey studies involving apolipoprotein-E include Schellenberg 1995, Mayeux and Schupf 1995, and the "Statement on use of apolipoprotein E testing for Alzheimer disease," published by the Working Group on ApoE and Alzheimer Disease, formed jointly by the American College of Medical Genetics and the American Society of Human Genetics, in the Journal of the American Medical Association 274: 1627-29 (1995). Review articles which survey studies of amyloid proteins or genes include Checler 1995, and Hendriks and Van Broeckhoven 1996. Articles which discuss the potential relationship between ApoE isoforms and amyloid plaque formation, or which provide general overviews of genetic testing in Alzheimer's disease, include Pericak-Vance and Haines 1995, Gearing et al 1996, and Polvikoski et al 1996.
At present, familial forms of Alzheimer's disease are recognized in which a genetic mutation that gives rise to the disease manifestations is inherited within families. By performing genetic analysis on members of such families, it is possible to identify, prior to onset of clinical symptoms, those members who have the genetic mutation and are, therefore, destined to develop the disease. Other forms of Alzheimer's disease (often referred to as "sporadic Alzheimer's disease") that do not show the familial inheritance pattern cannot be diagnosed with certainty, in advance, by detecting a single genotypic trait. Rather, it is thought that multiple risk factors act in concert to cause the disease in sporadic cases. An unfavorable apolipoprotein E allele is one such risk factor which can currently be identified by genetic analysis to permit identifying individuals who are at increased risk of developing sporadic Alzheimer's disease. As more risk factors become known, it will become possible to predict with increased certainty those individuals who are destined to develop sporadic Alzheimer's disease.
In genetic studies, the focus of research attention is mainly on trying to figure out how various genes and proteins (and possibly other factors) lead to amyloid plaque formation in the brains of Alzheimer patients. In general, genetic researchers studying Alzheimer's disease have shown little or no apparent inclination to incorporate the thinking of the excitotoxicity school into their hypotheses regarding the pathogenesis of neuronal degeneration in Alzheimer's disease. Currently, efforts by genetic researchers to explain how genetic factors can cause all of the neuropathological manifestations of Alzheimer's disease are at an impasse. The fact that amyloid plaques do not distribute in the same brain regions as the neurofibrillary tangles and degenerating neurons is a major contradiction that these researchers have not managed to resolve.
Excitotoxicity school
Other neurologists, who can be termed "excitotoxicologists," have maintained that the neurotoxic properties of glutamate may play a critical role in the neuropathological manifestations of Alzheimer's disease. One of the Applicants herein (Prof. John Olney) can be considered one of the founders of this school; the term "excitotoxicity" was first coined by Olney in the early 1970s.
The main hypothesis that has been embraced by the excitotoxicity school of thought (including the Applicants, until recently) is that neuronal degeneration in Alzheimer's disease results from an excitotoxic mechanism involving excessive activation (i.e., hyperactivation) of NMDA receptors; this condition is referred to herein as NR/hyper. This school originally proposed that an NR/hyper mechanism might be operative throughout all stages of Alzheimer's disease; this was their attempt to explain the neuronal degeneration occurring in all stages. In the late 1980's and early 1990's, a number of excitotoxicologists, including the Applicants, published review articles propounding this hypothesis (see, e.g., Henneberry et al 1989; Olney 1989; Albin and Greenemayre 1992; and Beal 1992).
Today, researchers in the excitotoxicity school have not abandoned this position, but they have encountered a problem with it, which they have not figured out how to resolve. The problem is that recent evidence documents that in the aging brain, the NR system is not in a hyperactive condition; rather, with advancing age it becomes hypoactive (i.e., less than normally active). If, as they originally proposed, the NR system in Alzheimer's disease were in a hyper condition, it would be appropriate to treat this condition with an NMDA antagonist drug to suppress NR and thereby correct the hyper condition. However, if the NR system in Alzheimer's disease is already in a hypo condition, treatment with an NMDA antagonist drug would not seem to make any sense, because there is not any NR/hyper condition to correct. Therefore, the excitotoxicology school can also be described as being at an impasse; they currently are not able to recommend an effective treatment approach for patients with Alzheimer's disease.
Very recently, the Applicants have made new discoveries that provide a solution to the above problem. Their recent findings have caused them to realize that NR/hyper is only a relevant mechanism to explain a certain limited pathological process that occurs very early in Alzheimer's disease, prior to the onset of outwardly detectable symptoms. They now realize that in this early period, the disease process is undergoing a transformation, in which the NR/hyper mechanism effectively "burns out" and destroys a number of NMDA receptors; this type of burnout damage leaves those damaged neural circuits in an NR/hypo condition, in which they can no longer adequately respond to normal levels of stimulation by glutamate. As described elsewhere, the damaged NR/hypo condition then becomes a disease mechanism capable of destroying certain target neurons in various regions of the brain.
Thus, the initial NR/hyper state undergoes a transformation into its exact opposite--an NR/hypo state. This apparently paradoxical opposite condition then becomes responsible for the widespread neurodegeneration that occurs in Alzheimer's disease.
Based on this new realization, a rational therapeutic program consisting of two sequential treatment approaches has now been recognized by the Applicants.
First, in the presymptomatic stages of the disease, an NMDA antagonist drug (plus a safener drug to prevent the neurotoxic side effects of the NMDA antagonist, unless the NMDA antagonist drug is inherently safe due to one or more additional receptor activities) is appropriate to prevent the NR/hyper process from burning out NMDA receptors and producing the NR/hypo state.
However, this is an appropriate approach only if instituted before the hyper-to-hypo shift at NMDA receptors has substantially taken place. After the shift has substantially occurred (i.e., after the NMDA receptor system has been damaged to a point where it is in a permanently NR/hypo condition), treatment with an NMDA antagonist would be contraindicated, because it would pose a risk of driving the NR system even deeper into a more profoundly NR/hypo state, thereby accelerating the second-stage type of damage that is caused by the NR/hypo state.
By the time outward symptoms of Alzheimer's disease become evident, some degree of neuronal damage has already begun to occur inside the brain. Based on the Applicants' recent discoveries, the onset of outward displays of Alzheimer-type damage is regarded by the Applicants as a transition point. At this transition point, an NR/hypo condition is presumed to have arisen inside the brain, and has initiated a brain damage process which, if not arrested, will cause increasingly more widespread degeneration of neurons throughout many cortical and limbic regions of the brain.
Accordingly, beginning at this transition point and continuing for the remaining course of the disease (which will be the remainder of the patient's life, in most cases), the appropriate treatment should avoid any use of NMDA antagonist drugs. Proper treatment, beginning after outward displays of Alzheimer-type damage have commenced, will use one or more of the drugs referred to herein as "safener drugs", which can counteract and prevent the type of brain damage caused by the NR/hypo mechanism, as evidenced by the ability of such drugs to block the toxic side effects of NMDA antagonist drugs such as phencyclidine or MK-801.
A transition point which is determined as described above, relying on the appearance of outwardly visible signs of the disease, offers a useful approximation to indicate when it becomes proper to withhold NMDA antagonist drugs and administer only safener drugs to a patient. However, it should be recognized that this is only an approximation. Now that the NMDA receptor mechanisms, and the sequence of damage-causing steps involved in Alzheimer's disease, have been described herein, neurologists are likely to develop more precise methods of determining when a condition of substantial NMDA receptor hypofunction has arisen in the brain of a specific patient. Methods for improving the accuracy and sophistication of determining the transition point for Alzheimer patients include neuroimaging methods such as PET scanning, magnetic resonance imaging, and magnetic resonance spectroscopy, and sophisticated cognitive tests, as described in articles such as Morris 1993 and Morris et al 1988.
Accordingly, one object of the present invention is to disclose a new model for understanding the pathology of Alzheimer's disease. This is a model that other neurologists studying Alzheimer's disease do not currently share, because their prior research has led them in different directions, and because they are unaware of certain items of relevant information being disclosed herein for the first time.
Another object of this invention is to disclose an appropriate treatment for Alzheimer's disease during its presymptomatic stages. During this very early stage of the disease, an NMDA antagonist drug should be administered, to prevent burnout of the NMDA receptors in the glutamate-driven control system. Unless the NMDA antagonist has a built-in safener activity (because of activity at a second class of neuronal receptors), the NMDA antagonist drug should be accompanied by a "safener" drug, to prevent the toxic side effects that would be caused by the NMDA antagonist drug in the absence of any safening activity.
Another object of this invention is to disclose a completely different treatment for Alzheimer's disease, after a substantial level of NMDA receptor damage has occurred and an NR/hypo condition has arisen in the brain. Once a patient reaches this stage, any NMDA antagonist drug should be terminated. Proper treatment to prevent subsequent damage to pyramidal and other "target" neurons discussed herein will consist of a safener drug alone. Such safener drugs, which can prevent neuronal damage caused by drug-induced NMDA receptor hypofunction, can also prevent neuronal damage caused by endogenous NMDA receptor hypofunction which is occurring as a disease process in the brains of patients who are suffering from Alzheimer's disease.
These and other objects of this invention will be clarified and explained in the following summary and description.